Engineering news
Duke University scientists have made conductive ink-jet printer "inks" by suspending tiny metal nanoparticles in liquids, in order to print inexpensive, customisable circuit patterns on practically any surface.
Printed electronics, which are already being used on a wide scale in devices such as radio frequency identification (RFID) tags, currently have one major drawback: for the circuits to work, they first have to be heated to melt all the nanoparticles together into a single conductive wire, making it impossible to print circuits on inexpensive plastics or paper.
A new study by Duke researchers shows that tweaking the shape of the nanoparticles in the ink can eliminate the need for heat.
By comparing the conductivity of films made from different shapes of silver nanostructures, the researchers found that electrons zip through films made of silver nanowires much easier than films made from other shapes, like nanospheres or microflakes. In fact, electrons flowed so easily through the nanowire films that they could function in printed circuits without the need to melt them together.
Electrons usually flow easily through individual nanostructures but get stuck when they have to jump from one structure to the next, and long nanowires greatly reduce the number of times the electrons have to make this "jump".
"The nanowires had a 4,000 times higher conductivity than the more commonly used silver nanoparticles that you would find in printed antennas for RFID tags," said Benjamin Wiley, assistant professor of chemistry at Duke. "So if you use nanowires, then you don't have to heat the printed circuits up to such high temperature and you can use cheaper plastics or paper."
These types of printed electronics could have applications far beyond smart packaging; researchers said the technology could also be used to make solar cells, printed displays, LEDS, touchscreens, amplifiers, batteries and even some implantable bio-electronic devices. The results appear online in ACS Applied Materials and Interfaces.
Ian Stewart, a recent graduate student in Wiley's lab and first author on the research paper, used known recipes to cook up silver nanostructures with different shapes, including nanoparticles, microflakes, and short and long nanowires, and mixed the nanostructures with distilled water to make simple "inks."
To make thin films in the lab researchers used a hole punch to cut out wells from double-sided tape and stuck these to glass slides. By adding a precise volume of ink into each tape "well" and then heating the wells – either to relatively low temperature to simply evaporate the water or to higher temperatures to begin melting the structures together he created a variety of films to test.
The team is now experimenting with using aerosol jets to print silver nanowire inks in usable circuits. They will also explore whether silver-coated copper nanowires, which are significantly cheaper to produce than pure silver nanowires, will give the same effect.